fix conditional compilation for net 5.0+ (#834)

Author: zhiweiv

src/KubernetesClient/Authentication/TokenFileAuth.cs

@@ -17,15 +17,15 @@ public TokenFileAuth(string tokenFile)
             TokenFile = tokenFile;
         }
 
-#if NETSTANDARD2_1_OR_GREATER
+#if NETSTANDARD2_1_OR_GREATER || NET5_0_OR_GREATER
         public async Task<AuthenticationHeaderValue> GetAuthenticationHeaderAsync(CancellationToken cancellationToken)
 #else
         public Task<AuthenticationHeaderValue> GetAuthenticationHeaderAsync(CancellationToken cancellationToken)
 #endif
         {
             if (TokenExpiresAt < DateTime.UtcNow)
             {
-#if NETSTANDARD2_1_OR_GREATER
+#if NETSTANDARD2_1_OR_GREATER || NET5_0_OR_GREATER
                 token = await File.ReadAllTextAsync(TokenFile, cancellationToken)
                     .ContinueWith(r => r.Result.Trim(), cancellationToken)
                     .ConfigureAwait(false);
@@ -40,7 +40,7 @@ public Task<AuthenticationHeaderValue> GetAuthenticationHeaderAsync(Cancellation
                 // < 10-8-1 minute.
                 TokenExpiresAt = DateTime.UtcNow.AddMinutes(1);
             }
-#if NETSTANDARD2_1_OR_GREATER
+#if NETSTANDARD2_1_OR_GREATER || NET5_0_OR_GREATER
             return new AuthenticationHeaderValue("Bearer", token);
 #else
             return Task.FromResult(new AuthenticationHeaderValue("Bearer", token));